Lydia is a senior in high school. She works 20 hours a week at a popular clothing store. She is trying to save as much money as possible for her college tuition. However, she has to buy a dress for prom, pay her car insurance, and get a birthday gift for her sister. This week she received a $400 paycheck. She wants to deposit the entire check in her savings account for her college tuition, but she stops herself. She decides instead to put $100 in the bank, spend $200 for her prom dress, pay $80 for her car insurance, and buy her little sister a $20 birthday gift. While shopping, she sees a doll worth $21, which she thinks her sister will absolutely love. As a result, she decided to buy the more expensive doll and spend less on her dress. How has Lydia shown that she was thinking marginally in buying her sister’s gift?